Bass synthesis is the collective name for a class of techniques that add in components to the low frequency range of an audio signal in order to enhance the bass that is perceived during playback of the enhanced signal. Some such techniques (sometimes referred to as sub bass synthesis methods) create low frequency components below the signal's existing frequency components in order to extend and improve the lowest frequency range. Other techniques in the class, known as “virtual pitch” algorithms, generate audible harmonics from an inaudible bass range (e.g., a bass range that is inaudible when the signal is rendered by small loudspeakers), so that the generated harmonics improve the perceived bass response. Virtual pitch methods typically exploit the well known “missing fundamental” phenomenon, in which low pitches (one or more low frequency fundamentals, and lower harmonics of each fundamental) can sometimes be inferred by a human auditory system from upper harmonics of the low frequency fundamental(s), when the fundamental(s) and lower harmonics (e.g., the first harmonic of each fundamental) themselves are missing.
Some virtual pitch methods are designed to increase the perceived level of bass content of an audio signal during playback of the signal by one or more loudspeakers (e.g., small loudspeakers) that cannot physically reproduce bass frequencies of the audio signal. Such methods typically include steps of analyzing the bass frequencies present in input audio and enhancing the input audio by generating (and including in the enhanced audio) audible harmonics that aid the perception of lower frequencies that are missing during playback of the enhanced audio (e.g., playback by small loudspeakers that cannot physically reproduce the missing lower frequencies). Such methods perform harmonic transposition of frequency components of the input audio that are expected to be inaudible during playback of the input audio (i.e., having frequencies too low to be audible during playback on the expected speaker(s)), to generate audible higher frequency components (i.e., having frequencies that are sufficiently high to be audible during playback on the expected speaker(s)). For example, FIG. 1 shows the frequency-amplitude spectrum of an audio signal, having an inaudible range 100 of frequency components, and an audible range of frequency components above the inaudible range. Harmonic transposition of frequency components in the inaudible range 100 can generate transposed frequency components in portion 101 of the audible range, which can enhance the perceived level of bass content of the audio signal during playback. Such harmonic transposition may include application of multiple transposition factors to each relevant frequency component of the input audio, to generate multiple harmonics of the component.